SUMMARYThe protein content of seeds determines their nutritive value, downstream processing properties and market value. Up to 95% of seed protein is derived from amino acids that are exported to the seed after degradation of existing protein in leaves, but the pathways responsible for this nitrogen metabolism are poorly defined. The enzyme pyruvate,orthophosphate dikinase (PPDK) interconverts pyruvate and phosphoenolpyruvate, and is found in both plastids and the cytosol in plants. PPDK plays a cardinal role in C 4 photosynthesis, but its role in the leaves of C 3 species has remained unclear. We demonstrate that both the cytosolic and chloroplastic isoforms of PPDK are up-regulated in naturally senescing leaves. Cytosolic PPDK accumulates preferentially in the veins, while chloroplastic PPDK also accumulates in mesophyll cells. Analysis of microarrays and labelling patterns after feeding 13 C-labelled pyruvate indicated that PPDK functions in a pathway that generates the transport amino acid glutamine, which is then loaded into the phloem. In Arabidopsis thaliana, overexpression of PPDK during senescence can significantly accelerate nitrogen remobilization from leaves, and thereby increase rosette growth rate and the weight and nitrogen content of seeds. This indicates an important role for cytosolic PPDK in the leaves of C 3 plants, and allows us to propose a metabolic pathway that is responsible for production of transport amino acids during natural leaf senescence. Given that increased seed size and nitrogen content are desirable agronomic traits, and that efficient remobilization of nitrogen within the plant reduces the demand for fertiliser applications, PPDK and the pathway in which it operates are targets for crop improvement.

SUMMARYC 4 photosynthesis occurs in the most productive crops and vegetation on the planet, and has become widespread because it allows increased rates of photosynthesis compared with the ancestral C 3 pathway. Leaves of C 4 plants typically possess complicated alterations to photosynthesis, such that its reactions are compartmented between mesophyll and bundle sheath cells. Despite its complexity, the C 4 pathway has arisen independently in 62 separate lineages of land plants, and so represents one of the most striking examples of convergent evolution known. We demonstrate that elements in untranslated regions (UTRs) of multiple genes important for C 4 photosynthesis contribute to the metabolic compartmentalization characteristic of a C 4 leaf. Either the 5¢ or the 3¢ UTR is sufficient for cell specificity, indicating that functional redundancy underlies this key aspect of C 4 gene expression. Furthermore, we show that orthologous PPDK and CA genes from the C 3 plant Arabidopsis thaliana are primed for recruitment into the C 4 pathway. Elements sufficient for M-cell specificity in C 4 leaves are also present in both the 5¢ and 3¢ UTRs of these C 3 A. thaliana genes. These data indicate functional latency within the UTRs of genes from C 3 species that have been recruited into the C 4 pathway. The repeated recruitment of pre-existing cis-elements in C 3 genes may have facilitated the evolution of C 4 photosynthesis. These data also highlight the importance of alterations in trans in producing a functional C 4 leaf, and so provide insight into both the evolution and molecular basis of this important type of photosynthesis.